Communication systems are known to include a plurality of communication devices and communication channels, which provide the communication medium for the communication devices. To increase the efficiency of the communication system, audio that needs to be communicated is digitally compressed. The digital compression reduces the number of bits needed to represent the audio while maintaining perceptual quality of the audio. The reduction in bits allows more efficient use of channel bandwidth and reduces storage requirements. To achieve audio compression, each communication device can include an encoder and a decoder. The encoder allows the communication device to compress audio before transmission over a communication channel. The decoder enables the communication device to receive compressed audio from a communication channel and render it audible. Communication devices that may use digital audio compression include high definition television transmitters and receivers, cable television transmitters and receivers, portable radios, and cellular telephones.
A subband encoder divides the frequency spectrum of the signal to be encoded into several distinct subbands. The magnitude of the signal in a particular subband may be used in compressing the signal. An exemplary prior art subband audio encoder is the International Standards Organization International Electrotechnical Committee (ISO/IEC) 11172-3 international standard, 20 Aug. 1991, hereinafter referred to as MPEG (Moving Picture Experts Group) audio. MPEG audio assigns bits to each subband based on the subband's mask-to-noise ratio (MNR). The MNR is the signal-to-noise ratio (SNR) minus the signal-to-mask ratio (SMR). The SMR is the signal level (SL) minus the masking level (ML). The SL, ML, SNR, SMR, and MNR are determined by a psychoacoustic unit. The psychoacoustic unit is typically the most complex element in an audio encoder, and the masking level calculation is typically the most complex element in a psychoacoustic unit. Also, the psychoacoustic unit is the most crucial element in determining the perceptual quality of an audio encoder, and the accuracy of the masking level calculation is crucial to the accuracy of the psychoacoustic unit.
Therefore, a need exists for a method, device, and systems that reduces the complexity of the masking level calculation while maintaining high perceptual quality in audio compression systems such as MPEG audio.